1. Field of the Invention
The present invention relates generally to a surface acoustic wave (SAW) apparatus. More particularly, the present invention relates to a surface acoustic wave filter (SAW resonator filter) configured as a resonator filter using a surface acoustic wave resonator (SAW resonator) or resonator having resonant characteristics in VHF and UHF bands.
2. Description of the Related Art
Recently, SAW devices are widely used in the telecommunications sector. Especially, SAW resonators or SAW resonator filters (hereinafter, referred to as SAW resonator devices) can be miniaturized and have lower loss and used a lot in mobile phones, remote keyless entry systems of automobiles and the like.
The SAW resonator device is constituted by at least one interdigital transducer (IDT) electrode disposed on a piezoelectric substrate and reflectors typically provided on both sides thereof. In the SAW resonator device, energy is confined between the reflectors by reflecting SAW propagated on the piezoelectric substrate through excitation of the IDT. On this occasion, a main propagation mode is a basic mode of modes distributing the energy vertically to the propagation direction of the SAW (hereinafter, referred to as transverse modes). On the other hand, second- or higher-order transverse modes also exist and, although electric charges excited by oscillations of even-order modes are canceled out, electric charges due to odd-order mode are not cancelled out and appear in resonator characteristics as spurious responses.
This spurious due to the higher-order transverse modes exerts a harmful influence as an oscillating-frequency skip phenomenon in an oscillation circuit in the case of the SAW resonator or an in-band ripple in the SAW resonator filter. The higher-order transverse modes are generated because an excitation intensity distribution of the SAW is in a rectangular shape.
FIG. 1 shows how the spurious is generated due to the higher-order transverse modes, using an example of a one-port SAW resonator with one (1) IDT 1 disposed between a pair of reflectors 2a, 2b. Although only a basic transverse mode 10a and a third-order transverse mode 10b are shown in the figure, fifth-, seventh- and higher-order transverse modes exist depending on an aperture length of the IDT.
In order to take a countermeasure for the spurious of the higher-order transverse modes, as shown in FIG. 2, it is known that the excitation intensity distribution 10 of the SAW is matched or approximated with the basic transverse mode 10a to suppress the higher-order transverse modes by using a COS function as overlapping-length weighting envelope curves to perform weighting for overlapping portions where a plurality of comb-shaped electrodes 1c connected to common electrodes 1a, 1b are interleaved (see., e.g., Japanese Examined Patent Application Publication No. 7-28195, Japanese Patent Application Laid-Open Publication Nos. 9-270667 and 7-22898).
In conventional examples, when the overlapping-length weighting envelope curves are looked along the direction of the surface acoustic wave, although the overlapping-length is gradually increased, the overlapping-length takes a downward turn at a certain changing point. This changing point exists as only one point.
The overlapping-length weighting envelope curves are mirror symmetry relative to an axis which is the propagation direction of the surface acoustic wave. Closely examining the symmetric property, for example, in a solid electrode configuration, a difference is generated which corresponds to a distance between adjacent electrode fingers, i.e., a ½ surface acoustic wavelength, however, not only in the solid electrode configuration, the ½ offset in the symmetric property of surface acoustic wavelength due to the electrode configuration is apparent from the structure of a surface acoustic wave apparatus, is not an essential portion of the present invention, and thus is not mentioned later while descriptions are made using the overlapping-length weighting envelope curves.
However, as shown in FIG. 2, if overlapping portions of comb-shaped electrodes 1c is weighted by a COS function as an overlapping-length weighting envelope curves, a distribution 11 is changed which is along the propagation direction of the overlapping-length of the comb-shaped electrodes of an IDT 1 and therefore, an excitation intensity distribution 10 of the SAW is also changed in the propagation direction.
Especially, the SAW resonator filter is affected significantly because the filter characteristics are achieved using a mode with a distribution in the propagation direction (longitudinal mode).
In the case of an IDT electrodes with uniform overlapping-length [normal(non-apodized) IDT electrodes], although a desired characteristic, for example, a bandwidth or an attenuation amount in a desired frequency can be achieved, spurious is generated due to the transverse mode. If the IDT electrodes are weighted by a COS function, although the spurious can be suppressed, a desired characteristic cannot be achieved very frequently.